Semiconductor-on-insulator structures are often used in applications where resistance to radiation effects on integrated circuits are required. Semiconductor-on-insulator structures include a layer of single crystal semiconductor, usually silicon, formed on an insulating structure such as sapphire or silicon dioxide. The single crystal layer is usually divided into separate regions or mesas to provide complete electrical isolation between the components formed in a mesa and components formed in other mesas. Semiconductor-on-insulator structures owe their robustness to radiation effects to the isolation of the electrical devices from the substrate. When a radiation particle, such as an alpha particle strikes a normal integrated circuit, the interaction between the electrical device and the substrate can cause errors in the operation of the integrated circuit. With semiconductor-on-insulator structures, this interaction is prevented by the insulating layer.
One problem presented by the use of semiconductor-on-insulator structures is the body effect on the channel of field effect transistors. Field effect transistors operate by coupling a voltage to a channel region which causes conduction between source and drain regions. The point at which the conduction between the source and drain occurs is called the threshold voltage. Because the channel is formed in an insulated structure, trapped charge can cause variations in the threshold voltage and thus inconsistencies in the operation of the field effect transistor, including uncontrolled turn on (conduction). It is common practice to provide a ground contact to the channel to prevent these effects. However, the effects of a floating channel region are not entirely negative. For example, a floating channel region can provide higher drive current through the channel region. This provides for faster operation of integrated circuits using transistors with higher drive currents. The present invention is directed to providing a structure and method to allow for a floating channel region during the "on" condition of the transistor when the higher drive current is useful and to provide a control voltage on the channel when the transistor is to be off to provide consistent threshold control.